The field of the invention is image display systems for nuclear or gamma cameras such as are used in the practice of nuclear medicine.
Nuclear imaging tomographic scanners such as those described in U.S. Pat. Nos. 4,652,758 and 4,497,024 include a sensor, or camera, which is sensitive to emissions from radioactive substances within the patient. The camera is typically moved around the patient and data is obtained from many angles. From the resulting data set a two-dimensional image can be reconstructed using well known computer tomographic (CT) techniques. Such two-dimensional images are cross sectional views through the patient and to obtain three-dimensional information from the data set, a series of spaced cross sectional views are reconstructed and the radiologist examines them all to reconstruct in his mind the three-dimensional image.
Three dimensional shaded surface images provide the radiologist with unique views of the radiopharmaceutical distributions within the patient. There are many known methods for producing such images from data sets collected using x-ray computerized tomography technology, nuclear magnetic resonance technology and nuclear imaging technology. These methods all require considerable computation time, and in the case of nuclear imaging technology, the images have not been particularly good because of the limited amount of data which is collected during a typical scan. This limitation on data translates into relatively low resolution 2-D images and inaccurate 3-D images.
The reconstruction of a 3-D shaded surface image may be accomplished in a number of well known ways. The first step is to extract from the data acquired throughout the scanned volume the data pertaining to the surface of the object of interest. This is usually accomplished either manually, or by a technique called thresholding. The thresholding technique identifies all those data points in the scanned volume where the nuclear events have exceeded a preset threshold value. Data points with lower numbers of events are considered background and the boundary between the background data points and the data points within the object of interest is the surface which is to be imaged.
After an object is identified, a mathmatical representation of the surface of the object is created using techniques such as the cuberille algorithm, contour-based algorithms, the octree-encoding algorithm or the ray-tracing algorithm. Surface display techniques are then employed to synthesize a 3-D image of the object. Such surface display techniques include hidden-surface removal, shading, translation, projection, scaling and rotation of the object of interest.
The ray-tracing method of creating a 3-D surface is particularly attractive since a separate pre-processing step is not required. With the so-called Vannier's method, for example, a projection plane perpendicular to the cross-sections of the object data set is established. Projectors perpendicular to the projection plane are traced toward the scanned volume. As a projector intersects a data point, or voxel, a thresholding test is applied to determine if that point is located on the surface of the object of interest. If the number of nuclear events exceeds the threshold value at the point, a point of the surface has been located and the projector stops. A shading value then assigned to the pixel in the plane from which the projector eminated. If the projection does not intersect the object of interest, its pixel shading value is set to the background value.
While the ray-tracing methods are particularly attractive with data acquired using x-ray computer tomography, the quality of the images has been less than satisfactory when such methods are applied to data sets acquired with nuclear imaging tomographic scanners. The primary reason for this is that fewer data points are acquired for a given volume of interest and reconstructed images have "staircase" artifacts.